Yes, but what does it actually mean?

Base rate neglect, the “99% accurate” test, and why a positive result often isn’t what you think Here is a question that has caught out more doctors than anyone in the medical profession would like to admit. There’s a disease that affects 1 in 1,000 people. There’s a test for it, and the test is…

Correlation vs causation, the ice cream that never drowned anyone, and the difference between “moves together” and “makes it happen” Here is a genuinely true fact. On days when more ice cream is sold, more people drown. The two rise and fall together with impressive reliability. Chart ice cream sales against drownings across a year…

Goodhart’s Law, the cobra bounty, and what happens when a measure becomes a target Somewhere in your house, possibly on your wrist right now, there may be a device counting your steps. Ten thousand a day is the number, and it is a perfectly reasonable stand-in for “move your body a bit more”. Right up…

Survivorship bias, the bombers that came home, and the data you never get to see “They don’t make them like they used to.” You have heard it, and you may even have said it, usually while admiring a solid old wooden dresser or a building that has stood for two hundred years. The craftsmanship was…

Simpson’s paradox, the Berkeley admissions row, and how the overall number can tell the opposite story Let’s start with two colleagues. Priya and Tom both handle enquiries. The enquiries come in two kinds: easy ones, which mostly get resolved, and hard ones, which mostly don’t. On the easy ones, Tom has the better record: he…

Averages, graduate salaries, and the number that nobody actually earns Imagine a bar with ten people in it. Nine of them are having a perfectly ordinary Tuesday – nurses, teachers, office workers, a couple of students. Between them they earn an average of around £30,000 a year. Then Bill Gates walks in. The average salary…